The aim of this investigation is to identify, solubilize, purify and characterize the phosphate carrier of mammalian mitochondria. Unpublished evidence is presented that the phosphate carrier may be identified among the proteins of beef heart mitochondrial inner membrane by selective labeling with radioactive N-ethyl meleimide (NEM) followed by sodium dodecylsulfate gel electrophoresis. Using the information gained on the properties of the NEM-labeled protein, we propose the design of experiments to isolate and purify, by chromatographic and electrophoretic methods, the phosphate carrier in its free, uninhibited form. The carrier will be examined for phosphate- binding capacity. Attempts to document evidence for a carrier function for the phosphate-binding protein will include incorporation of the protein into liposomes and testing for phosphate transport as well as inhibition of phosphate transport in mitochondrial inner membrane particles by antibody to the phosphate-binding protein.